They said that no one should be expected to lie about their allegiances in order to avoid persecution, and that it would in any case be impossible for judges to decide if their pretence would be believed.

The ruling allowed four Zimbabweans to remain in Britain, defeating attempts by the Home Office to deport them, but could be used more widely by those claiming asylum to avoid being returned to dictatorships even if they have never been dissidents.

Donna Covey, the Chief Executive of the Refugee Council, said: “We welcome this important judgment, confirming that the right to political freedom includes the right not to support any particular political party.

“People should not have to lie about having or not having political beliefs as a way of avoiding persecution.”

But Sir Andrew Green, chairman of Migration Watch UK, said: “This is ridiculous. It is yet a further extension of the grounds on which asylum may be granted.

“We already have a serious problem in that 60 per cent of applications turn out to be bogus and a similar proportion don’t bother claiming until they are discovered.

“It is time the courts realised the extent to which the asylum system is being exploited.”

Wednesday’s ruling centred on the case of a 31-year-old Zimbabwean woman who claimed she held no strong political beliefs but faced being tortured, raped or killed back home if she could not prove her support for President Robert Mugabe's Zanu PF.

The woman, known only as RT, came to Britain on a student visa but later had her asylum claim rejected by officials who argued that she could simply pretend that she backed the ruling party. According to her lawyer, she was told to "go home and lie" about her political neutrality.

She lodged a case with the Court of Appeal which agreed that, if she were returned to Zimbabwe and interrogated about her politics, she should not be expected to lie. The Home Office appealed to the Supreme Court but it has now ruled in her favour.

Her lawyer cited an earlier judgment in which the justices ruled that a homosexual Iranian man should not be expected to return home and avoid ill-treatment by being discreet, as suggested by the Home Office.

The Supreme Court justices agreed that the same rules applied, adding that there were "no hierarchies of protection" amongst the Refugee Convention reasons for persecution.

In a unanimous judgment, seven justices agreed that a person’s right not to support any political party deserved the same protection under the United Nations' 1951 Refugee Convention as the right to support them.

They said that the woman, and three others involved in the case, would be at risk from local militias who had "delivered a quite astonishingly brutal wave of violence" in areas where people were unable to prove that they supported Zanu PF by carrying the party card or singing its campaign songs.

“Persecution on the grounds of imputed opinion will occur if a declared political neutral is treated by the regime as a supporter of its opponents and persecuted on that account,” the senior judges said.

Lord Kerr added in the judgment that there was an “all too real and predictable” risk of persecution in the “extreme, repressive and anarchic” conditions of Zimbabwe, and that it was “deeply unattractive if not indeed totally offensive” to expect people to lie to avoid the violence.

It would be an “impossible exercise” for an immigration judge to decide if any individual could convince an “undisciplined and unpredictable militia” of their support for Zanu PF.

The decision will make it harder in future for the Home Office to deport Zimbabweans, as many as 1million of whom have settled in Britain since the increase in political violence and collapse of Zimbabwe's economy over the past 15 years.

Britain suspended enforced returns to its former colony in 2006 because of the unstable political situation but they resumed in June 2010 after the formation of an inclusive government between Morgan Tsvangirai's Movement for Democratic Change and Mr Mugabe's Zanu PF.